Preliminary results are encouraging for the achievement of high conversion efficiencies using a GaAsP top solar cell mechanically stacked on a conventional silicon solar cell. A realistic maximum of 29.4 percent is suggested when both the top and bottom solar cells are state of the art. Practical system efficiencies greater than 25 percent are attainable in the near future with the use of a state of the art bottom solar cell

Barnett, Allen M.

Gartley, Edgar A.

Lasswell, Patrick G.

Mcneely, James B.

Negley, Gerald H.

English

NASA Technical Reports Server

N87-26420
MECHANICALLY-STACKED TANDEM SOLAR CELLS WITH GaAsP ON GaP AND SILCON*
Gerald H. Negley, James B. McNeely, Patrick G. Lasswell, and Edgar A. Gartley
Astrosystems, Inc.
Newark, Delaware
and
Allen M. Barnett
University of Delaware
Newark, Delaware
The development of GaAsP top solar cells for mechanical attachment to
silicon bottom solar cells can lead to AM0 conversion efficiency increases of 48%
to 76% over the best state-of-the-art single junction silicon solar cells. These
tandem solar cells can also be expected to be more radiation-resistant and
mechanically and electrically stable.
Design rules are presented for the development of a high efficiency tandem
stack. The system efficiency can range from 26.7% to 29.4% depending on the
performance of the bottom solar cell. Consideration of the near term goal of a
25% efficient tandem solar cell is addressed. Guidelines for the achievement of
this near term goal are given in terms of device parameters.
Liquid phase epitaxy is being used for this development of GaAsP on GaP top
solar cells. Considerable progress has been demonstrated in the liquid phase
epitaxial growth of GaAsP on GaP substrates. Multiple graded layers of GaAsP
with up to 65% GaAs have been prepared with surface quality equivalent to
commercial GaP on GaP epitaxial wafers. Techniques for stacking fault and
dislocation density reduction are being developed. High quality active layers
have been prepared with lattice parameters that differ from the GaP substrate by
2.41%.
The first experimental two-junction, four-terminal tandem cells with a GaAsP
top solar cell on a conventional silicon bottom solar cell have been fabricated.
Top solar cell transmission of 95% of the photons less energetic than the top
cell bandgap has been accomplished. Initial results for these tandem solar cells
will be presented. Future work will focus on increasing current in the top cell
and increasing the device area.
*This work was supported in part by the Air Force AeroPropulsion
under contract No. F33615-86-C-2605.
Laboratory
55
https://ntrs.nasa.gov/search.jsp?R=19870016987 2020-03-20T09:51:14+00:00Z
Introduction
The addition of a GaAsP on GaP top solar cell over existing silicon solar
cells has the potential of increasing the power output of a practical system by
48 to 76%. This is dependent on the bottom solar cell performance. The tandem
solar cell efficiency can range from 26.7 to 29.4%. In this paper the design
rules for the achievement of these performance levels are discussed. Next, those
performance requirements necessary to achieve the near term goal of 25% are
addressed. These performance goals can be described in terms of transparency,
voltage, fill factor and current. Liquid phase epitaxy is being used for the
growth of GaAsP on GaP. The progress in the development of a GaAsP top solar
cell is presented.
Desi_
The design of the mechanically stacked GaAsP on GaP top cell is based on a
model used to calculate theoretical maximum efficiencies of tandem solar cell
systems. The model that is being used is by Nell (i) and is based upon tabulated
standard spectra, the fit of experimentally achieved open-circuit voltages, and
asst_nes unit quantum efficiency.
Using solar irradiance information, the performance is calculated for the
top solar cell. The remaining part of the spectrum, E < Eg (top), is then used
to calculate the performance of the bottom cell. In this way, a complete set of
isoefficiency curves is generated for various energy bandgap combinations.
Assuming unit quantum efficiency and no losses, the model predicts a maximum
solar cell efficiency of 35.8% at AMO and one sun insolation. This performance
is based upon a four-terminal configuration for the tandem stack.
The maxim_ theoretical efficiency of 35.8% corresponds to a 1.97eV top
solar cell and a 1.12eV bottom solar cell This is equivalent to a GaAs _aP a_
top cell and a conventional silicon bottom'solar cell. The device parameters _
the "ideal" tandem stack are shown in Table I.
Table I
Predicted Theoretical Maximum Efficiency for the
Four-Terminal Configuration according to Nell's Model
Bandqap Voc Jsc FF Efficiency
(eV) (volts) (_-/-cm2) -- (%)
1.97 1.53 20.75 .91 21.4
1.12 .71 32.68 .84 14.4
35.8
Anticipated Performance for a Practical System
To obtain the maximum efficiency for a practical system both the top and
bottom solar cells must be state-of-the-art. The overall efficiency of the
tandem stack must include electrical and optical losses. Loss calculations for
56
this tandem structure have been anticipated by others (2). In this work, the
performance of other types of solar cells is used as the basis for predicting
feasible system performance.
To obtain the maxim_npractical efficiency, both the top and bottom solar
cells must be approaching their theoretical limit. The maximumefficiency
predicted by the model for a silicon space solar oell is 23.9% at AMOand one sun
(i). Swanson's (3) record efficiency concentrator solar cell is equivalent to
19.9% efficient for an AM9spectrum at one sun, while Green's (4) best results
correspond to an efficiency of 18.6% (AMO,I sun). These results are 22%to 31%
better than the average conmercial silicon space solar cells. Table II shows a
comparison of these results.
Table II
Modelled Theoretical Maximum for Silicon Space
Solar Cells Compared to Actual Silicon Results
Voc Jsc FF Efficiency
(volts) (_--2--cm2) -- (%)
Model .710 53.43 .840 23.9
Swanson .681 50.30* .784 19.9
Green .663 45.40* .833 18.6
Conm_rcial .595 46.00 .750 15.2
*Corrected from AMi.5G to AMO
The modelled theoretical efficiency of the 1.97eV top solar cell is 21.4%.
Again, losses must be included to obtain the best "real" case. Since the
GaAs _P _ top solar cell is still in the experimental stages, tabulation of the
limi£_pre_cted by the model with the best results to date does not allow a fair
comparison of the overall tandem stack. However, if one surveys the literature
and compares the performance of well-developed solar cells with their expected
limit, one may easily predict the expected performance of the 1.97eV GaAsP top
cell. Mid-range achievements of open-circuit voltage, short-circuit currents and
fill factors are 96%, 91%, and 96%, respectively, of the expected limits from the
model. Using these assumptions, the best case 1.97eV GaAsP top solar cell should
peak at 17.9% efficiency. This is shown in Table III.
Table III
Expectation of Best Case GaAs.54P.46
(i. 97eV)
Voc Jsc FF Efficiency
(volts) (mA--/--om2) (%)
Model 1.53 20.75 .91 21.4
Best Case 1.47 18.90 .87 17.9
% Theoretical 96% 91% 96%
57
With the expectations of the top and bottom solar cells, one can view the
performance of the GaAsP-Si tandem stack. The best case tandem structure with
various bottom solar cells is shown in Table IV. The tandem stack efficiencies
range from 29.4%to 26.7% depending on the bottom solar cell.
Table IV
Best Case Tandem Solar Cell with
Various Silicon Bottom Solar Cells
(AMO)
Swanson Green Conlnercial
GaAsP Top Cell
Si bottom cell
Stack Efficiency (%)
17.9 17.9 17.9
11.5" 10.8" 8.8*
29.4 28.7 26.7
losses.
*Includes an extra 5% reduction in Jsc due to optical transmission
Requirements for a 25% Efficient Tandem Solar Cell
The near term goal is a 25% efficient solar cell at AM0. An inherent
requirement to achieve this goal is the utilization of a state-of-the-art bottom
solar cell. From Table II, the choice is the 19.9% efficient solar cell produced
by Swanson. The performance requirements necessary for achievement of a 25%
tandem device can be described in terms of transparency, voltage, fill factor and
current.
The transparency can be determined once the choice of the energy bandgap for
the top solar cell is made. From solar spectral irradiance data (5), one
determines the portion of the spectrum absorbed in the top solar cell. The
remaining part of the spectrum may be utilized by the bottom solar cell.
However, due to loss mechanisms, such as free carrier absorption, the actual
transmitted light may be less than that predicted by the solar spectral irra-
diance data. The ratio of the actual transmitted light to the theoretical
maximL_n predicted by the irradiance data gives the overall transparancy of the
material for photons less energetic then the bandgap.
The transparency of the top solar cell determines the overall reduction in
performance of the bottom solar cell. This reduction is in the short-circuit
current. Allowing for an additional 5% loss due to optical losses, the best
state-of-the-art silicon solar cell should be 11.5% efficient when placed under a
GaAs P top solar cell. Hence, a 13.5% efficient top solar cell is needed to
..54 .46
achleve 5he goal of a 25% efficient tandem stack.
To achieve a 13.5% efficient top solar cell, an onpen-circuit voltage of 1.46
volts, a short-circuit current density of 14.9 mA/cm _ and a fill factor of 0.84
are needed. This short-circuit current density corresponds to a total quantum
efficiency of 71.8%.
A comparison of the best case GaAsP solar cell parameters to those required
for the achievement of a 25% efficient tandem solar cell is shown in Table V.
58
Table V
Comparison of Best Case GaAsP to Requirements
for a 25% Efficient Tandem Solar Cell
Voc Jsc FF Efficiency
(volts) (_-2-cm2) -- (%)
Best Case 1.47 18.9 .87 17.9
Required 1.46 14.9" .84 13.5
*Requires total quant_n efficiency of 71.8%.
Proqress of GaAsP Top Cell Device Fabrication
The GaAsP solar cell structure is being grown by liquid phase epitaxy
(LPE). Liquid phase epitaxial crystal growth has, in general, produced devices
that are superior in performance to those gr(mm by other methods (6, 7). The
superior performance of LPE devices when compared to vapor phase or diffused
devices can be attributed to fewer deep level impurities, longer diffusion
lengths and the fact that the impurities tend to segregate to the liquid rather
than the solid.
The actual crystal growth of the multi-layer GaAsP on GaP structure is
achieved using the slider method for LPE growth (8). The slider apparatus serves
as a substrate holder and melt container for the growth solutions. Advantages of
the slider method over other techniques are i) the substrate can be brought in
and out of contact with the melts, 2) several melts can be used, 3) growth is
restricted to a single side of a wafer, 4) substrate - solution contact is from
the bottom of the melts where there are no floating oxides or other contaminants,
5) excess solution can be wiped off by the sliding action of the apparatus, and
6) thermal equilibration and temperature profiling are easily facilitated. The
slider assembly fits into a temperature gradient or cooling furnace. The zones
of the furnace are controlled to better than l°C. Currently, in our furnace, we
use a high purity hydrogen atmosphere which sweeps the growth apparatus and tube
during the growth process.
The slider assembly contains up to nine melts and consists of a graded well
design such that each successive layer is narrower than the former layer. This
can be seen in Figure i. This allows testing and analysis of each individual
layer. Individual melts are composed of approximately eight grams of solvent
with appropriate amounts of GaP and GaAs - determined by phase equilibria data
for the compositions desired. Growth is achieved by placing the GaP substrate
under the first melt to grow a transition layer of GaASxPl_ x by controlling the
temperature level, cooling rate, and time of exposure. This procedure,
continues, in turn, to each melt in the growth apparatus. Since GaP segregates
preferentially over GaAs in metallic systems, melt depletion can be used to grade
the layers from pure GaP to the desired final layer composition.
59
Fiqure 1
Slider Boat Growth Apparatus Showing
the Graded Well Width
r,._ C_ P SU bST RATt=
Multiple graded structures of GaAsP with up to 65% GaAs have been grown in
our laboratories with surface qualities equivalent to commercial GaP on GaP
epitaxial wafers. Currently, a combination of melt depletion and step grading
has produced the best surface morphologies. This consists of a 10% GaAs layer
grown on the GaP substrate. This 10% GaAs layer is depleted to approximately 12%
GaAs before moving the substrate to the next melt for another step grade. The
following step is to a 15% GaAs layer which, in turn, is depleted to about 17%
GaAs. Then, the substrate is contacted to a 20% GaAs melt and melt depletion is
used to grade to about 40% GaAs. At this point, the substrate is contacted with
another melt to achieve the final GaAsP composition. Since melt depletion is
used to grade the final two layers, the ending composition is determined by the
final temperature. Modelling has been done to anticipate the temperature at
which the final desired composition will be achieved. Table VI shows various
expected vs. measured compositions.
Table VI
Comparison of Expected vs. Measured
Composition of Grown GaAsP
Sample Design Energy Gap
% GaAs Expected Measured
(ev)
PT#66 50 2.01 2.01
PT#101 47 2.03 2.01
PT#104 79 1.69 1.69
PT#117 65 1.85 1.88
PT#193 54 1.97 1.98
PT#198 54 1.97 1.98
PT#200 54 1.97 1.96
PT#209 54 1.97 1.97
6O
Since the LPE boat assembly has a graded well design (which allows easy
access to the individual grown layers), optical transmission is used to determine
the compositions of the grown layers. A typical structure determined by optical
transmission is shownin Figure 2.
Fiqure 2
OPTICAL TRANSMISSION OF FOUR LAYER STRUCIURE
I
°!
°i
/
A: GaP
B: GaAs.12P.8 8
C: GaAs.17P.8 3
D: GaAs.46P.5 4
E: GaAs.54P.4 6
The preferred solvent for the GaAsP-GaP solvent system is galli_n. When tin
is u_d as a solvent, the grown layer is n-type with a carrier concentration of
6xl0"°/cc, which is too high for our solar cell design.
We are currently working on p/n structures, with the preparation of the thin
emitter layers being accomplished by solid state diffusion techniques. This
process was developed in our laboratory and is very similiar to diffusion from
spin-on silica glasses (9). However, our process consist of a SiO 2 passivation
layer to protect the crystal surface, a Zn3P 2 layer for the zinc source, and a
SiO 2 capping layer for the entire structure. The SiO 2 enhances the diffusion of
a relatively low concentration of zinc (thereby mini/izing wafer surface damage
while facilitating high surface concentrations (i0)). These diffusions are
leading to open-circuit voltages in excess of 1.4 volts; however, the short-cir-
cuit current has been low.
Figure 3 shows the spectral response of two of our devices compared to a
GaAs standard. Sample PT#184 has a 1.84eV composition, which shows a low quant_n
yield and a flat response. It is believed that part of this low yield is due to
a dead layer resulting from the solid state diffusion process.
Sample H041886 has a 2.08eV composition and shows a more peaked response. This
sample peaks in the region corresponding to the direct bandgap. Hence, some
efficiency is lost in the indirect composition region. The poor blue response of
H041886 is most likely due to a deep junction.
61
Fiqure 3
SPECTRAL RESPONSE OF TWO GaAsP DEVICES
LO
O.l
0.6
0*4.
0.2. //_
0.0 /'
P'r_k
ws,,,_s_ Czm)
i t/,
To achieve the 71.8% quantum efficiency needed for the attainment of a 25%
tandem solar cell, the flat response of PT#184 must be brought up to the level of
the H041886 peak. We are currently investigating the spectral response of MIS
devices and grown p/n junctions in order to achieve this.
The near term goal of a 25% efficient tandem solar cell requires a 13.5%
efficient top solar cell. Table VII shows our best parameters to date.
Table VII
Best GaAsP on GaP Solar Cell Parameters
Tarqet Actual
Transparency below Eg (%) 95 92.3"
Voc (volts_ 1.46 1.43
Jsc (mA/cm _) 14.9 14.6"*
FF .84 .84
*Single layer AR coating
**MIS Device
The first experimental two-junction, four-terminal tandem cells with GaAsP
top solar cell on a conventional silicon bottom cell has been prepared. The top
cell has an open circuit voltage of 1.397 volts and a fill factor of 0.81,
although the current was low. Nonetheless, the tandem stack outperformed the
conventional silicon solar cell by more than 10%. If the GaAsP top solar cell
were stacked on a state-of-the-art silicon solar cell, an efficiency approaching
20% would have been achieved. A transparency of 95.0% has been achieved with
this tandem structure - this includes grid shading but neglects busbar losses.
Conclusions
Preliminary results are encouraging for the achievement of high conversion
efficiencies using a GaAsP top solar cell mechanically stacked on a conventional
silicon solar oell. A realistic maximum of 29.4% is suggested when both the top
and bottom solar cells are state-of-the-art. Practical system efficiencies
62
greater than 25%are attainable in the near future with the use of a state-of-
the-art bottom solar cell.
References
(i) M.E. Nell and A.M. Barnett, "The Spectral p-n Junction Model for Tandem
Solar Cell Design", Conf. Rec. 18th IEEE Photovoltaic Specialist Conference,
116, Las Vegas (1985).
(2) A.M. Barnett, M.G. 5_uk, J.C. Zolper, I.W. Hall, W.A. Tiller, J.B. McNeely
and R.B. Hall, "Thin Film Silicon and GaAs Solar Cells", Conf. Rec. 17th
IEEE Photovoltaic Specialist Conference, 747, Orlando (1984).
(3) R.A. Sinton, Y. Kwark, J.Y. Gan, and R.M. Swanson, "27.5% Silicon Concen-
trator Solar Cells", IEEE Electron Device Letters, EDL-7 (#i0), 567-569,
1986.
(4) M.A. Green, A.W. Blakers, S.R. Wenham, S. Narayanan, M.R. Willison,
M. Taouk, and T. Szpitalak, "Improvements in Silicon Solar Cell Efficiency",
Conf. Rec. 18th IEEE Photovoltaic Specialist Conference, 39, Las Vegas
(1985).
(5) NASA Space Vehicle Design Criteria (Environment), Solar Electromagnetic
Radiation, NASA SP-8005, May 1971.
(6) L.R. Dawson, '_iquid Phase Epitaxy", Proqress in Solid State Chemistry,
Chapter 4, 7j 117, Pergamon Press (1972).
(7) R.L. Moon, "Liquid Phase Epitaxy", Chapter ii, Crystal Growth, ed. B.Pamp-
fin, 2nd ed., p.421, Pergamon Press (1980).
(8) H. Nelson, "Depositing Successive Epitaxial Layers from the Liquid Phase",
U.S. Patent 3,565,702 issued February 23, 1971.
(9) Emulsitone Co., "Deposition of Doped and Undoped Silica Films on Semicon-
ductor Surfaces", U.S. Patent 3,615,943 issued October 26, 1971.
(i0) Filmtronics, Inc., "Semiconductor Diffusant Products", processing informa-
tion, p.35.
63


Mechanically-stacked tandem solar cells with GaAsP on GaP and silicon

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